1. Field of Invention
The invention relates to a method and apparatus for operating a fuel cell system and, more specifically, to a method and apparatus for operating a fuel cell system in response to an electrical power demand by an electrical load.
2. Description of the Related Art
As disclosed in German Patent No. DE 195 40 824 C2, one of the problems encountered in providing electrical power from a fuel cell is related to the supply of oxidant to the fuel cell itself. In this respect, the oxidant supply compressor may be powered exclusively by an electric motor (as in the above-mentioned patent document), or may be powered by an expander, which expands exhaust gases that have been generated in the fuel cell and utilizes the resulting recovered thermal energy to drive the compressor. The latter design is known, for example, from German Patent No. DE 197 55 116 C1.
A disadvantage of such systems is that the power needed by the compressor is generated by the fuel cell itself, either as electrical power or as energy recovered from the exhaust gases. This creates a problem with regard to the dynamic response, as the fuel cell is not able to immediately provide increased electrical energy since the necessary increase in oxidant volume must first be provided to the fuel cell by the compressor.
To address this dynamic response limitation, it has generally been accepted to use an electrical energy storage device to provide the temporary energy shortfall to the electrical load that is demanding a certain target power. In a motor vehicle, such demand is typically based on the position of the accelerator pedal. For motor vehicle applications, this means that it is necessary to employ electrical storage devices, such as batteries, which are able to provide comparably high power and a correspondingly high amount of energy. Batteries of this nature are typically very large and heavy, which is a significant drawback in motor vehicle applications in terms of weight and space requirements.
Accordingly, there is a need in this field for a method and apparatus for operating a fuel cell system in response to an electrical power demand by an electrical load that overcomes the above-mentioned dynamic response limitation. The present invention fulfills this need and provides further related advantages.